Restore support for Visual Studio 2010 and Visual Studio 2012
Address TPL faults for .NET 4
Fix issues with IDisposable
Merge pull request #43 from sharwell/vs2010

Restore support for Visual Studio 2010 and Visual Studio 2012
Merge pull request #47 from sharwell/disposable-pattern

Fix issues with IDisposable
Update README.md
Refactor EditorDiffMargin.UpdateDiffDimensions to simplify visibility handling
Remove unnecessary code duplication
Duplicate UpdateDiffDimensionsImpl in preparation for separate handling of removed blocks
Simplified code for handling "normal" hunks
Simplified code for handling "deleted" hunks (fixes #44)